++
Spinal anesthesia (SA) is one of the simplest and most reliable of
regional anesthesia techniques. The external anatomic landmarks are easily
identified. The block can be performed with minimal discomfort, the
end-point of cerebrospinal fluid flow is unmistakable, and the onset of
anesthesia is more rapid than with any other regional technique. Because of
the rapidity of onset, the block can be performed in the operating room
without the requirement for additional personnel or a block room. The
efficient performance of the block does not add substantially to operating
room time any more than for the induction of GA. The onset of the sensory
blockade is sufficiently rapid to attain surgical anesthesia by the time the
positioning and preparation of the patient are completed. A variety of local
anesthetic agents are available that can provide a wide range of duration of
surgical anesthesia. The risk of nausea can be reduced if systemic opioids
are avoided. Likewise, nausea and vomiting2,9,10 and
residual somnolence associated with general anesthetics or heavy
premedication can be avoided, allowing a rapid return to full alertness in
the PACU. SA is also a technique with a high degree of patient familiarity
because of its use in obstetrics and thus is more likely to be accepted by
many patient populations. In addition, it employs the lowest dose of local
anesthetic of any major regional anesthetic technique and has minimal
potential for systemic toxicity.
++
Epidural anesthesia (EA) shares many of the advantages of SA,
particularly clinician familiarity, simplicity of superficial landmarks, and
ease of performance of the block. It has the additional advantage of
allowing a continuous catheter to be placed in the epidural space, which
creates the potential for tailoring both the segmental spread and duration
of the block. Although it is a more flexible technique, this advantage is
attained at the price of a slower onset of surgical anesthesia.
+++
Combined Spinal–Epidural Anesthesia
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Combined spinal–epidural anesthesia (CSEA) is also a useful technique
in the outpatient setting. The procedure is technically more challenging:
once the epidural space is identified, the spinal needle must be introduced
through the epidural needle and advanced further into the subarachnoid
space. After the local anesthetic is injected, the spinal needle is
withdrawn and the epidural catheter is inserted into the epidural space and
taped in place. This technique requires more time and technical skill, but
CSEA provides the advantages of the rapid onset and dense block of SA along
with the flexibility of an indwelling catheter to allow incremental and
repeated injections to achieve the desired segmental spread and duration of
surgical anesthesia. This technique has been used effectively for
extracorporeal shock wave lithotripsy procedures, where the duration of
treatment may be unpredictable. Another advantage of the combined technique
is the rapid onset of dense perineal anesthesia, which may not be provided
by lumbar EA alone. It has also been used for knee arthroscopies when low
doses of subarachnoid local anesthesia are used to provide a predictable
short duration, but may be supplemented by epidural injection of local
anesthetic if further spread or duration of blockade are
needed.11 Although the technique combines some of the
disadvantages of both neuraxial procedures, it also maximizes the advantages
and positive aspects of both SA and EA.
++
Neuraxial anesthesia does have potential disadvantages. SA is typically
a single-injection blockade, and thus careful attention must be paid to
selection of the appropriate local anesthetic agent and dose. If the
surgical duration was underestimated or becomes prolonged for unexpected
reasons, supplemental GA may be needed. The “single-injection” aspect of
SA frequently induces clinicians to give “just a little bit more” drug to
ensure adequate distribution and duration; however, this tendency must be
resisted, as the downside of this increased dosing pattern is a prolonged
recovery and discharge time.6
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Postdural puncture headache (PDPH) remains a risk with SA. Newer
pencil-point, smaller gauge needles have significantly reduced this
frequency to less than 3%.12 It is even less frequent
in patients older than age 40. Although PDPH does not result in long-term
neurologic damage and usually is not a prolonged inconvenience for the
patient, it must be acknowledged in the discussion of SA with the
outpatient. If it is inconvenient for a patient to return for an epidural
blood patch or essential that the patient not have this debility,
alternative anesthetic techniques should be considered.
++
The most recent concerns about SA for outpatients have revolved around
potential toxicity of local anesthetic drugs. A major concern was the
reporting of permanent neurologic damage associated with very high doses of
concentrated lidocaine injected through spinal microcatheters. This has not
been a problem with standard doses of the local anesthetics used for
single-injection spinal anesthetics, although all anesthetics injected in
the subarachnoid space are potentially neurotoxic.13 A
more common, relevant concern has been the symptoms of neurologic irritation
associated primarily with lidocaine. This syndrome of “transient neurologic
symptoms” (TNS) consists of a burning type of back pain radiating into the
buttocks or legs that appears 6–24 h after the resolution of SA and can
persist for 1 to 6 days.14 TNS occurs approximately
15–30% of the time, with the highest frequency following lidocaine
SA.15 Obese outpatients are more susceptible, especially
those having procedures performed in the lithotomy or knee arthroscopy
positions.16 Although no sensory or motor deficits are
associated with this syndrome,17 and to date no persistent
neurologic deficits, it is nevertheless a significant source of morbidity in
some patients. Many practitioners have sought alternatives to lidocaine to
reduce the incidence of TNS (see section on Lidocaine).
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EA also has some potential drawbacks in the outpatient setting. Its slower
onset of blockade compared with SA may cause a slight delay. However, if
induction of EA is performed in a block room outside the operating room, the
onset of anesthesia with drugs such as chloroprocaine (2-CP) or lidocaine is
so rapid that there is little delay in the onset of surgery, and the use of
EA may even promote operating room efficiency.18 Other
drawbacks associated with EA include the greater risk for postdural headache
if an unintentional dural puncture occurs. Since the potential for headache
is directly related to the size of the needle, the use of the larger gauge
epidural needles may represent a greater risk, although the incidence of
PDPH from an unintentional dural puncture in experienced hands should be
less than 0.5%. EA involves the use of larger doses of local anesthetic
drugs, and thus represents a greater potential for systemic toxicity than
occurs with SA. The careful use of safety steps is just as appropriate for
the outpatient as for the inpatient.19
++
Another limitation of both neuraxial techniques is the absence of residual
analgesia. Multiple randomized comparisons of neuraxial techniques to GA
show that early pain is significantly less with the regional
techniques,1–3,20,21 but once the block has resolved,
some alternative mode of analgesia must be provided for the patient. This
may be accomplished by the use of local anesthetic infiltrated into the
wound, intraarticular injection of local anesthetic, or even a supplemental
peripheral nerve block (eg, a femoral nerve or ankle block). The need for
additional analgesia may not be an issue after relatively less painful
procedures, such as diagnostic knee arthroscopy. Nevertheless, the
possibility for breakthrough pain must be considered in the planning of the
central neuraxial anesthesia for the outpatient.
++
A common concern is the potential for difficulty with urination following
neuraxial blockade. With higher doses of longer acting local anesthetics,
the bladder is distended beyond its normal cystometric capacity during the
prolonged duration of neural blockade and may be unable to return to normal
function once the sensory blockade dissipates.22
Fortunately, with the short-duration central neuraxial blockades that are
usually employed in the outpatient setting, bladder function returns
promptly following complete resolution of the blockade. Patients can be
successfully discharged home after short-duration spinal anesthetics with
procaine, 2-CP, lidocaine, and even low doses of
bupivacaine.23 The use of certain additives, such as
epinephrine, may impede this recovery.24,25 The
requirement for postoperative voiding is not essential with short-acting
local anesthetics or low dose (<6 mg) bupivacaine spinal anesthetic
techniques.